1. Field of the Invention
This invention relates to a current sensor for detecting an electric current flowing through a signal line. More particularly, the present invention relates to a high sensitivity current sensor for detecting an electric current by using a magneto-impedance device or a flux-gate sensor whose magnetic permeability changes in response to the external magnetic field and also to a current detection unit using such a current sensor.
2. Related Background Art
Conventionally, current transformers and hall devices are used as current sensors for detecting an electric current. The current sensor market is expected to expand as electric automobiles have come on the scene and power monitoring for the purpose of power saving has been promoted in recent years. Current detecting operations have been increasingly ramified as a result of advancement of current management using networks and accordingly the detection range of current detecting operations has been lowered. As a result, there is an increasing demand for highly sensitive current sensors. In short, as the detection range of current detecting operations lowers, the magnetic sensors to be used for the detecting operations are required to show a higher S/N ratio.
In such a trend, the use of magneto-impedance devices and flux-gate sensors that are more sensitive than conventional hall devices is highly promising. Then, it is important to realize a compact current sensor by using such a sensitive device.
FIG. 9 of the accompanying drawings shows a known typical current sensor that comprises a magnetic circuit by a C-shaped magnetic core 102 turned around an electric wire 100 and a hall device 104 arranged in the gap of the magnetic core 102. The hall device 104 does not show saturation and hence can operate in a strong magnetic field because it is not ferromagnetic.
However, magneto-impedance devices and flux-gate sensors are formed by using a ferromagnetic substance and hence require to be used within a non-saturation range because they show magnetic saturation. In other words, when a magnetic circuit is configured by using a magneto-impedance device or a flux-gate sensor like the one using a hall device, the magnetic field of the gap becomes too strong and magnetic saturation appears easily if an electric current in the order of amperes is detected. Thus, a circuit configuration like that of a circuit using a hall device is not feasible when a magneto-impedance device or a flux-gate sensor is involved.
Additionally, it is necessary for a high sensitivity magnetic sensor, when it is ferromagnetic, to confine the magnetic field detection range to about ±15 gausses (±1.5 milliteslas or mT) if a wide detection range can be realized by using negative feedback or the like for the drive circuit. In other words, when the magnetic field from an electric wire is to be detected directly, the distance needs to be adjusted so that intensity of the magnetic field may be found within the above detection range.
However, the current sensor may confront dimensional limitations if the combination of a device and an electric wire is switched from one to another depending on the required current detection range. Then, the applicability of such a current sensor will also be limited. Particularly, a current sensor adapted to large electric currents may need to have large dimensions.
Furthermore, the magnetic fields produced from some electric parts other than the target electric wire that may include power transformers need to be handled appropriately. If a magnetic shield is used, conditions for using such a shield need to be considered so that the shield may not influence the magnetic field produced from the electric wire nor be magnetically saturated.